Network Working Group Ned Freed
Internet Draft Keith Moore
<draft-freed-pvcsc-01.txt>
MIME Parameter Value and Encoded Words:
Character Sets, Language, and Continuations
November 1996
Status of this Memo
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1. Abstract
This memo defines extensions to the RFC MIME-IMB media type
and RFC 1806 disposition parameter value mechanisms to provide
(1) a means to specify parameter values in character sets
other than US-ASCII,
(2) to specify the language to be used should the value be
displayed, and
(3) a continuation mechanism for long parameter values to
avoid problems with header line wrapping.
Internet DrafIME Parameter Values and Encoded Wordovember 1996
This memo also defines an extension to the encoded words
defined in RFC MIME-HEADERS to allow the specification of the
language to be used for display as well as the character set.
2. Introduction
The Multipurpose Internet Mail Extensions, or MIME [RFC-MIME-
IMB, RFC-MIME-IMT, RFC-MIME-HEADERS, RFC-MIME-REG, RFC-MIME-
CONF], define a message format that allows for
(1) textual message bodies in character sets other than
US-ASCII,
(2) non-textual message bodies,
(3) multi-part message bodies, and
(4) textual header information in character sets other than
US-ASCII.
MIME is now widely deployed and is used by a variety of
Internet protocols, including, of course, Internet email.
However, MIME's success has resulted in the need for
additional mechanisms that were not provided in the original
protocol specification.
In particular, existing MIME mechanisms provide for named
media type (content-type field) parameters as well as named
disposition (content-disposition field). A MIME media type
may specify any number of parameters associated with all of
its subtypes, and any specific subtype may specify additional
parameters for its own use. A MIME disposition value may
specify any number of associated paramters, the most important
of which is probably the attachment disposition's filename
parameter.
These parameter names and values end up appearing in the
content-type and content-disposition header fields in Internet
email. This inherently imposes three crucial limitations:
(1) Lines in Internet email header fields are folded
according to RFC 822 folding rules. This makes long
parameter values problematic.
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(2) MIME headers, like the RFC 822 headers they often
appear in, are limited to 7bit US-ASCII, and the
encoded-word mechanisms of RFC MIME-HEADERS are not
available to parameter values. This makes it
impossible to have parameter values in character sets
other than US-ASCII without specifying some sort of
private per-parameter encoding.
(3) It has recently become clear that character set
information is not sufficient to properly display some
sorts of information -- language information is also
needed [RFC-IAB-CHARSETS]. For example, support for
handicapped users may require reading text string
aloud. The language the text is written in is needed
for this to be done correctly. Some parameter values
may need to be displayed, hence there is a need to
allow for the inclusion of language information.
The last problem on this list is also an issue for the encoded
words defined by RFC MIME-HEADERS, as encoded words are
intended primarily for display purposes.
This document defines extensions that address all of these
limitations. All of these extensions are implemented in a
fashion that is completely compatible at a syntactic level
with existing MIME implementations. In addition, the
extensions are designed to have as little impact as possible
on existing uses of MIME.
IMPORTANT NOTE: These mechanisms end up being somewhat
gibbous when they actually are used. As such, use of these
mechanisms should not be used lightly; they should be reserved
for situations where a real need for them exists.
3. Parameter Value Continuations
Long MIME media type or disposition parameter values do not
interact well with header line wrapping conventions. In
particular, proper header line wrapping depends on there being
places where linear whitespace (LWSP) is allowed, which may or
may not be present in a parameter value, and even if present
may not be recognizable as such since specific knowledge of
parameter value syntax may not be available to the agent doing
the line wrapping. The result is that long parameter values
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may end up getting truncated or otherwise damaged by incorrect
line wrapping implementations.
A mechanism is therefore needed to break up parameter values
into smaller units that are amenable to line wrapping. Any
such mechanism must be compatible with existing MIME
processors. This means that
(1) the mechanism must not change the syntax of MIME media
type and disposition lines, and
(2) the mechanism must not depend on parameter ordering
since MIME states that parameters are not order
sensitive. Note that while MIME does prohibit
modification of MIME headers during transport, it is
still possible that parameters will be reordered when
user agent level processing is done.
The obvious solution, then, is to use multiple parameters to
contain a single parameter value and to use some kind of
distinguished name to indicate when this is being done. And
this obvious solution is exactly what is specified here: The
asterisk character ("*") followed by a decimal count is
employed to indicate that multiple parameters are being used
to encapsulate a single parameter value. The count starts at
0 and increments by 1 for each subsequent section of the
parameter value. Decimal values are used and neither leading
zeroes nor gaps in the sequence are allowed.
The original parameter value is recovered by concatenating the
various sections of the parameter, in order. For example, the
content-type field
Content-Type: message/external-body; access-type=URL; URL*0="ftp://";
URL*1="cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar"
is semantically identical to
Content-Type: message/external-body; access-type=URL;
URL="ftp://cs.utk.edu/pub/moore/bulk-mailer/bulk-mailer.tar"
Note that quotes around parameter values are part of the value
syntax; they are NOT part of the value itself. Furthermore,
it is explicitly permitted to have a mixture of quoted and
unquoted continuation fields.
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4. Parameter Value Character Set and Language Information
Some parameter values may need to be qualified with character
set or language information. It is clear that a distinguished
parameter name is needed to identify when this information is
present along with a specific syntax for the information in
the value itself. In addition, a lightweight encoding
mechanism is needed to accomodate 8 bit information in
parameter values.
Asterisks ("*") are reused to provide the indicator that
language and character set information is present and encoding
is being used. A single quote ("'") is used to delimit the
character set and language information at the beginning of
the parameter value. Percent signs ("%") are used as the
encoding flag, which agrees with RFC MIME-HEADERS.
Specifically, an asterisk at the end of a parameter name acts
as an indicator that character set and language information
may appear at the beginning of the parameter value. A single
quote is used to separate the character set, language, and
actual value information in the parameter value string, and an
percent sign is used to flag octets encoded in hexadecimal.
For example:
Content-Type: application/x-stuff;
title*=us-ascii'en'This%20is%20%2A%2A%2Afun%2A%2A%2A
Note that it is perfectly permissible to leave either the
character set or language field blank. This is done when
either character set, language, or both are not relevant to
the parameter value at hand. This MUST NOT be done in order
to indicate a default character set or language -- parameter
field definitions MUST NOT assign a default character set or
lanugage.
4.1. Combining Character Set, Language, and Parameter
Continuations
Character set and language information may be combined with
the parameter continuation mechanism. For example:
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Content-Type: application/x-stuff
title*1*=us-ascii'en'This%20is%20even%20more%20
title*2*=%2A%2A%2Afun%2A%2A%2A%20
title*3="isn't it!"
Note that:
(1) Language and character set information only appear at
the beginning of a given parameter value.
(2) Continuations do not provide a facility for using more
than one character set or language in the same
parameter value.
(3) A value presented using multiple continuations may
contain a mixture of encoded and unencoded segments.
(4) The first segment of a continuation MUST be encoded if
language and character set information are given.
(5) If the first segment of a continued parameter value is
encoded the language and character set field delimiters
MUST be present even when the fields are left blank.
5. Language specification in Encoded Words
RFC MIME-HEADERS provides support for non-US-ASCII character
sets in RFC 822 message header comments, phrases, and any
unstructured text field. This is done by defining an encoded
word construct which can appear in any of these places. Given
that these are fields intended for display, it is sometimes
necessary to associate language information with encoded words
as well as just the character set. This specification extends
the definition of an encoded word to allow the inclusion of
such information. This is simply done by suffixing the
character set specification with an asterisk followed by the
language tag. For example:
From: =?US-ASCII*EN?Q?Keith_Moore?= <moore@cs.utk.edu>
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6. IMAP4 Handling of Parameter Values
IMAP4 [RFC-1730] servers SHOULD decode parameter value
continuations when generating the BODY and BODYSTRUCTURE fetch
attributes.
7. Modifications to MIME ABNF
The ABNF for MIME parameter values given in RFC MIME-IMB is:
parameter := attribute "=" value
attribute := token
; Matching of attributes
; is ALWAYS case-insensitive.
This specification changes this ABNF to:
parameter := regular-parameter / extended-parameter
regular-parameter := regular-parameter-name "=" value
regular-parameter-name := attribute [section]
attribute := 1*attribute-char
attribute-char := <any (US-ASCII) CHAR except SPACE, CTLs,
"*", "'", "%", or tspecials>
section := initial-section / other-sections
initial-section := "*1"
other-sections := "*" (("2" / "3" / "4" / "5" /
"6" / "7" / "8" / "9") *DIGIT) /
("1" 1*DIGIT))
extended-parameter := (extended-initial-name "="
extended-value) /
(extended-other-names "="
extended-other-values)
extended-initial-name := attribute [initial-section] "*"
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extended-other-names := attribute other-sections "*"
extended-initial-value := [charset] "'" [language] "'"
extended-other-values
extended-other-values := *(ext-octet / attribute-char)
ext-octet := "%" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
charset := <registered character set name [RFC-CHARSET-REG]>
language := <registered language tag [RFC-1766]>
The ABNF given in RFC MIME-HEADERS for encoded-words is:
encoded-word := "=?" charset "?" encoding "?" encoded-text "?="
This specification changes this ABNF to:
encoded-word := "=?" charset ["*" language] "?" encoded-text "?="
8. Security Considerations
This RFC does not discuss security issues and is not believed
to raise any security issues not already endemic in electronic
mail and present in fully conforming implementations of MIME.
9. References
[RFC-822]
Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", RFC 822 August, 1982.
[RFC-1730]
Crispin, M., "Internet Message Access Protocol - Version
4", RFC 1730, December 1994.
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[RFC-1766]
Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March, 1995.
[RFC-1806]
Troost, R. and Dorner, S., "Communicating Presentation
Information in Internet Messages: The Content-
Disposition Header", RFC 1806, June 1995.
[RFC-MIME-IMB]
Freed, N. and Borenstein, N., "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC MIME-IMB, Innosoft, FIrst Virtual Holdings,
June 1996.
[RFC-MIME-IMT]
Freed, N. and Borenstein, N., "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC MIME-IMT,
Innosoft, First Virtual Holdings, June 1996.
[]
Moore, K., "Multipurpose Internet Mail Extensions (MIME)
Part Three: Representation of Non-ASCII Text in Internet
Message Headers", RFC MIME-HEADERS, University of
Tennessee, June 1996.
[RFC-MIME-REG]
Freed, N., Klensin, J., Postel, J., "Multipurpose
Internet Mail Extensions (MIME) Part Four: MIME
Registration Procedures", RFC MIME-REG, Innosoft, MCI,
ISI, June 1996.
[RFC-MIME-CONF]
Freed, N. and Borenstein, N., "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC MIME-CONF, Innosoft, FIrst Virtual
Holdings, June 1996.
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[RFC-CHARSET-REG]
Freed, N. and Postel, J., "IANA Character Set
Registration Procedures", Innosoft, ISI, draft-freed-
charset-reg-02.txt, November 1996.
[RFC-IAB-CHARSETS]
Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
Atkinson, R., Crispin, M., Svanberg, P., "Report from the
IAB Character Set Workshop", Version 3.0, November 1996.
10. Authors' Addresses
Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
tel: +1 818 919 3600 fax: +1 818 919 3614
email: ned@innosoft.com
Keith Moore
Computer Science Dept.
University of Tennessee
107 Ayres Hall
Knoxville, TN 37996-1301
USA
email: moore@cs.utk.edu
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